High density printed circuit boards are typically constructed of several electrically conductive layers separated by layers of dielectric material. Some of the conductive layers may be utilized as power and ground planes while other conductive layers may be patterned for electrical signal connections (e.g. between integrated circuit chips). Layer-to-layer interconnections may be used in such constructions and accomplished using what is referred to as plated-through-holes (PTH's), such holes typically including a plating of electrically conductive material (e.g., copper) therein. In those situations wherein electrical interconnections are desired between adjacent conducting layers, it has been common in the art to provide such connections with what are commonly referred to as "vias". These hole type connections, though typically not extending through the entirety of the board's thickness, are also coated (e.g., plated) with an internal conductive layer (e.g., copper). Such "vias" and through-holes are typically provided by drilling.
Examples of various types of multilayer printed circuit board constructions are defined in U.S. Pat. Nos. 4,030,190 (K. Varker), U.S. Pat. No. 4,554,405 (K. Varker), U.S. Pat. No. 4,854,038 (J. Wiley), U.S. Pat. No. 4,864,772 (D. Lazzarini et al) and U.S. Pat. No. 4,868,350 (J. Hoffarth et al). All of these patents are assigned to the same assignee as the instant invention.
As defined in the above and other patents, and as is also well known in the art, such multilayered printed circuit board constructions typically utilize copper or a similar highly conductive material for the signal and/or power and/or ground conductive planes. One well known example of a dielectric material for use in such constructions is epoxy glass. Other materials may include polyimide and polytetrafluoroethylene (e.g., Teflon, a trademark of E.I. du Pont de Nemours and Company), the latter of more recent vintage when relatively low dielectric materials are desired.
As will be defined herein, the present invention comprises a multilayered circuit board construction comprised of individual layered subassemblies each including electrically conducting wiring and at least one through-hole therein. The resulting multlilayered structure as defined herein is characterized by the provision of a first one of these layers possessing a substantially high wiring density and at least one other layer possessing a substantially lesser current resistance in comparison to the other layers in the structure. By the term wiring density as used herein is meant to define the number of linear inches of conductive wiring per square inch of area per individual layer within each subassembly. These wiring conductors may be used for signal conducting, power and/or ground functions, depending on operational requirements. Thus, a layered subassembly having one conductive layer with four separate and spaced conductive lines each of one inch length and all located in a one inch square total area would possess a wiring density of 4.0 in/in.sup.2.
By the term resistance as used herein is meant to define the amount of electrical resistance encountered by the electrical current passing through the individual conductive wiring in each layered subassembly. In the above example, this would be the amount of resistance (in ohms per inch) of the four lines of circuitry in the subassembly.
Use of layered subassemblies in an overall structure wherein selected ones of such subassemblies possess a substantially greater wiring density than other subassemblies while said other subassemblies possess significantly less resistance to current passing therethrough (and corresponding reduced wiring density) results in a multilayered structure of substantial versatility. Such a structure may include several (e.g., ten) individual layers as part thereof with those on one side of the board possessing relatively high wiring densities (and corresponding relatively low current capacity) while those of the other side of the board possess less resistance to the currents which flow therethrough but which also possess relatively low wiring densities. Such a structure is particularly useful when maximum input (e.g., signals) is desired to one side of the board and much greater current flow is desired within the other side. In this other side, such greater current flow (and associated lesser resistance), e.g., as provided by substantially larger (e.g., thicker) conductive wiring which may be spaced apart at greater distances than the smaller and more closely spaced conductive wiring in the side of the board possessing the aforedefined greater densities, enables not only the opportunity for greater current capacities than said other side but also for longer wiring paths between desired connections. Such a structure is considered particularly useful in supercomputer environments.
Accordingly, it is believed that a multilayered circuit board assembly possessing the above advantageous features would constitute a significant advancement in the art.